Scientists have developed semiconductor lasers with two frequency combs

Scientists at the Federal Institute of Technology in Zurich, Switzerland, have adopted a new approach to producing highly stable skew-frequency comb pairs from a single, integrated freewheeling semiconductor stack laser (Science, doi: 10.1126 / science.aam7424). Using birefringent crystals in the laser cavity, they can tune and separate the beam from the frequency comb offset simply by changing the crystal thickness. The team said: "This drastically reduces the complexity of dual comb spectroscopy." This emerging technology leads to a wider range of industrial applications.

The EPFL team at Zurich University produced two coherent offset bursts using the same laser pulse train of birefringent crystals to generate a pair of optical frequency combs that could drive the double comb spectrum of a gas sample. Use heterodyne detection to extract dual comb signals in the microwave domain.

Seek stability

In 2005, the Nobel Prize in physics was awarded a frequency comb spectrum that accurately measured atoms and molecules as a scale using spectra generated by hundreds of thousands or even millions of equally spaced, sharp-edged lasers. In the double comb method, adding a second frequency comb as a reference significantly improves the scan rate and spectral resolution of the method (see "Dual-comb spectroscopy", OPN, January 2017). In both cases, the comb sources are typically femtosecond-level and mode-locked lasers whose bursts in the time domain appear as closely spaced optical combs in the frequency domain.

However, the great challenge of practical application of the double comb method lies in the fact that the two coherent phase-stable bursts that can be realized in the laboratory can not be easily applied to the industrial environment because of their free running laser. Some double comb devices overcome this difficulty by using real-time, computational error correction to achieve the necessary stability between the two combs.

Another way to create two stable frequency combs that are coherent with each other is to generate double combs from the same free-running laser. In this manner, two bursts that produce a paired frequency comb share a cavity such that the combs are coherent and stable to each other without interference from phase locking or post-event error correction. The research team has now demonstrated some laser single-laser dual comb array devices; for example, in a study reported in 2016, a circular pulse with a back-propagation laser mode and a nonlinear crystal were used to generate the offset pulse sequence (T). Illguchi et al., Optica, doi: 10.1364 / OPTICA.3.000748).

MIXSEL way

The ETH team led by OSA researcher Ursula Keller wanted to verify that it could obtain a single laser dual comb array by using an optically pumped semiconductor lamination laser, an obvious and compact platform based on wafer-level technology, Its application to mass production and more widely used. In particular, the researchers designed zeroing on a mode-locked integrated external cavity surface-emitting laser (MIXSEL). In MIXSEL, a saturable absorber (for mode locking) and gain media are integrated into one and the same semiconductor chip; the chip forms a mirror at one end of the laser cavity and an output coupler at the other end. External Diode Laser Pump MIXSEL Cavity.

Inside the MIXSEL cavity, the ETH also places another element: birefringent crystals. The crystal divides the MIXSEL source into two co-linear bursts by polarization; in addition, the crystal is arranged to satisfy the difference required for the pulse repetition frequency between the two pulse trains due to the difference in the optical path lengths of the two trains passing through it.

The results verify that two collinear optical frequency combs interfere at the photodetector. As with other dual-comb combinations, the actual measurements are made using heterodyne detection with two THz frequency combs mixed to produce a single microwave frequency comb; the mixed signal encodes the broadband, high-resolution spectral signals of the two optical combs into a fast readable , Perfect RF digital signal processing beat frequency.

Water test

The team tested the device in a water vapor double comb spectroscopy experiment and achieved fast, good spectral analysis using a stable, free-running laser. Studies have shown that the simple error correction / stability of the hybrid microwave comb increases the signal-to-noise ratio properties and stability further and can be adjusted by adjusting the thickness of the birefringent crystal (hence the difference in optical path length) Poor frequency.

According to the team, one disadvantage of this system is that the frequency combs generated by MIXSEL have a relatively limited bandwidth. This means that analyzing different molecular species with different resonance frequencies, for example, may require different MIXSEL lasers.

The researchers point out that the bandgap engineering techniques that have been established in the semiconductor industry make the center wavelength of these MIXSEL-based systems relatively easy and cheap to implement. As a result, researchers believe semiconductor lamination laser arrays with specific operating wavelengths can be more compact and cost-effective than a single expensive tunable Ti: sapphire laser. "We believe that the dual comb MIXSEL method has the potential to bring dual comb spectra from the lab environment to the field for a wide range of industrial applications."

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